Prior art novolac resist materials generally use a novolac resin and a photosensitive agent as two main components. Numerous studies have been made on the novolac resin, photosensitive agent and solvent in order to develop a resist material which is improved in sensitivity, resolution, pattern geometry, heat resistance, film retention, adhesion-to-substrate, and shelf stability, and which can accommodate the wavelength of a light source in various steppers.
With respect to the novolac resin, for example, resist characteristics are largely affected by molecular weight fractions of a novolac resin. For example, a tandem type novolac resin consisting of a low molecular weight fraction (Mw=150 to 500) and a high molecular weight fraction (Mw&gt;5,000), with a middle molecular weight fraction (MW=500 to 5,000) excluded, provides a resist composition having a good balance of sensitivity, resolution and heat resistance (see T. Kajita et al., Proc. SPIE, 1446, 161 (1991)).
With respect to the photosensitive agent, research works comply with the change of the stepper light source from g-line to i-line. Since resist compositions comprising conventional benzophenone photosensitive agents have a low transmittance, it was proposed to use non-benzophenone photosensitive agents having less absorption of i-line (see Nikkei Microdevice, April 1992, page 45).
Research works have also been made on the solvent. A resist composition is typically prepared using a conventional cellosolve solvent such as ethyl cellosolve acetate. When it is allowed to stand, even after filtration through a filter with a pore size of 0.2 .mu.m, very fine microparticulates which are not visible to the naked eyes can form in the resist composition. Some microparticulates have a size of more than 0.5 .mu.m. If a resist pattern of about 1 .mu.m is formed on a wafer using the resist composition containing such relatively large microparticulates, the microparticulates are left on the pattern, leading to a lower resolution and exacerbating the manufacture yield of integrated circuits. In this regard, it is proposed in JP-B 3-22619, for example, to formulate a resist composition having improved long-term storage stability by dissolving an alkali-soluble resin and a 1,2-quinonediazide compound in a monooxycarboxylate-containing solvent.
These approaches dealing with the novolac resin and photosensitive agent have drawbacks. In order to realize a high resolution resist composition of good performance by combining the above measures, many additional steps are needed for the resist composition-formulating process, adding to the cost. The approach focusing at the solvent also has drawbacks. Insofar as a 1,2-quinonediazide compound monomer is used as the photosensitive agent in a resist composition as described above, the monomer will precipitate after filtration, detracting from the long-term shelf stability of the resist composition. A further improvement is thus desired.